(a) Field of the invention
The present invention relates to a cata-dioptric objective lens system and, more particularly, to a compact and light-weight cata-dioptric objective lens system.
(b) Description of the prior art
As cata-dioptric telephoto objective lens systems have an annular aperture, it is necessary in most cases to provide an opening at the central portion of the first reflecting surface, i.e., the primary mirror. The opening of the primary mirror should be worked so that eccentricity will not be caused between the center of the opening and optical axis of the primary mirror and that the accuracy of the reflecting surface will not decrease and it is very difficult to work the opening satisfactorily. As a cata-dioptric objective lens system in which the primary mirror is not provided with an opening, the cata-dioptric telephoto objective lens system according to U.S. Pat. No. 3,632,190 is known. However, in case of the lens system according to U.S. Pat. No. 3,632,190 undesirable ghosts will be caused by the rays which pass through the central portion of the primary mirror on which the first reflecting surface is formed and, therefore, it is necessary to provide a cylinder for preventing the detrimental rays in the lens system and to thereby prevent the detrimental rays from passing through the central portion of the primary mirror. But, it is considerably difficult to fix such cylinder in position. Besides, to compactly arrange the lens system as a whole, it is effective to some extent when the first reflecting surface is arranged to have a strong converging effect and the second reflecting surface is arranged to have a strong diverging effect. However, it is unavoidable to place the second reflecting surface at a position about 1/2 of the total optical path length (the optical path length from the front surface of the first lens component to the film surface along the optical path) of the lens system. To make the overall length of the lens system short, it is therefore necessary to make the converging effect of the first reflecting surface extremely strong. In that case, however, coma will be aggravated considerably.